This invention relates generally to a multipole rod assembly for guiding or separating ions in a mass spectrometer assembly.
One type of quadrupole mass spectrometer which is well known in the art employs a plurality of spaced parallel rods or elements with suitable AC and/or DC voltages applied between pairs of the rods to generate shaped electric fields in the area between the rods. A beam of charged particles to be analyzed is injected into the electric field at one end of the rods. The electric fields serve to filter the particles whereby only particles of selected mass-to-charge ratio exit from the other end. The mass-to-charge ratio of the particles which are passed is determined by the voltages applied between the rod pairs. The particles exiting the rods are detected and an output signal is generated. By scanning the voltages, the output signal represents a spectrum of the mass-to-charge ratio of the particles present in the input beam. Linear multipole rod assemblies are also used as quadrupole ion traps for Fourier transform quadrupole mass spectrometers such as described in U.S. Pat. No. 4,755,670. Linear multipole rod assemblies are also used as ion guides.
In quadrupole mass spectrometers, and in ion trap mass spectrometers, the selectivity and sensitivity of the mass spectrometer is dependent upon the accuracy of spacing of the rods in both the circumferential and radial direction along the length of the rods. In the past, rods have been supported by quartz or fused silica mounts located near each end of the rods. The quartz or fused silica mounts are machined to provide grooves which receive the rods. The rods are secured to the quartz or fused silica mounts by screws which extend through holes in the quartz or fused silica mount and into tapped holes in the rods. Machining required both for the quartz or fused silica mount and the tapped holes is relatively expensive and is critical to the proper mounting of the rods. Pairs of rods are then interconnected by metal straps or conductors.
U.S. Pat. No. 4,990,777 describes a multipole rod assembly in which the rods are supported in an aligned relationship by ceramic rings which carry metal rings with brackets supporting the rods from the ceramic rings. U.S. Pat. No. 5,852,294 describes a miniature multipole rod assembly which can be operated as an ion guide or a mass analyzer which is constructed by bonding individual rods directly to plates which are separated by ceramic insulators. The multipole rod assemblies are constructed by using a fixture which locates and orients all elements during the process or bonding the rods to the disks.
Iinear multipole rod assemblies may comprise quadrupole, hexapole and octopole assemblies. All of the prior art multipole rod assemblies are relatively expensive to manufacture with the required critical accuracy of alignment and spacing of the rods in the radial and circumferential direction.
It is an object of the present invention to provide an improved multipole rod assembly for multipole mass spectrometers.
It is another object of the present invention to provide a multipole rod assembly which includes an improved rod mounting and connection assembly.
The foregoing and other objects of the present invention are achieved by a rod assembly in which rectangular rods with shaped facing surfaces are embedded in dimensionally stable supports made of insulating material.